1. Field of the Invention
This invention relates to a temperature history indicator, that is, an indicator which undergoes a detectable change to an extent which is a function of temperature integrated over time.
2. Description of the Prior Art
Instantaneous temperature indicators are known, such as thermocouples, but although these are very accurate, it is complicated and expensive to use them to record the temperature history of a workpiece.
Peak temperature indicators are known, consisting for example of a crayon formulated to have a given melting point. When a workpiece marked with the crayon reaches this temperature, the mark at once melts, leaving an irreversible signal that the temperature was reached. Such an indicator is however unsuitable where transient temperature fluctuations are unimportant and it is required to determine a long-term average temperature, for example for lifetime assessment purposes for boiler components.
A temperature history indicator called "Templug" was developed by Shell Research Ltd. as a means of assessing temperatures in experimental internal combustion engines, where conventional methods of temperature measurement were impossible. The underlying physical principle depends on the fact that some metals and alloys undergo a permanent change in hardness if they are subjected to high temperatures. At a given temperature the amount of change in hardness will depend on the time of exposure. Conversely, if the time of exposure is known, measuring the hardness change can give an indication of the temperature at which it operated. The main advantages of using Templugs are versatility (they can be made into virtually any shape and size), durability (they have no fragile or delicate parts and are ideally suited to measure temperatures in extremely hostile environments), and cheapness.
However, even the Templug has some disadvantages. Thus, measurement of temperatures using Templugs involves taking hardness measurements on Templug samples. This often requires performing the measurement in a laboratory as portable hardness testers are less reliable.
The time required to perform one hardness measurement can be as long as one minute.
Hardness measurement is semi-destructive, in that once a measurement has been made it cannot be repeated over exactly the same spot.